Multi-level analog signaling (MAS) is used in Ethernet (10 Gigabit Ethernet) and other applications. Various MAS techniques include T-Waves, Quadrature Amplitude Modulation (QAM) and, of most interest to this invention, PAM, in particular PAM-5. In general, transmitting different amplitude levels over a serial asynchronous link can be used to reduce electromagnetic interference and other problems, and is a well known technique.
If an oscillator is required at the receiver for data recovery, then the ability to reduce receiver power consumption during idle periods (static power consumption) is compromised, as the oscillator will typically remain powered on at least for some part of the idle period. If powered down or off, then some finite amount of time is required to re-power and settle the oscillator circuitry when the idle period ends (i.e., when data reception begins again). Further, and depending on the architecture of the system, there may be a plurality of instances of the receiver circuitry, each requiring its own associated oscillator. As may be appreciated, in many applications it is desirable to minimize power consumption, circuit complexity and cost. While the clock signal could be transmitted through a separate line from the transmitter to the receiver, this technique also adds cost and complexity to the system. For example, 4-level logic (with a separate clock line) is used in, for example, RAMBUS memory systems, with an option to use only the two middle amplitude levels.
A publication of interest to the teachings of this invention is IEEE Journal of Solid State Circuits, Vol 29, No 9, September 1994: Crister Svensson and Jiren Yuan, “A 3-Level Asynchronous Protocol for a Differential Two-Wire Communication Link”. This publication describes a technique that uses multi-level amplitude signaling in such a way that there is no need to provide an oscillator at the receiver. In the 3-level signaling method of Svensson et al. the symbol 0 is represented by a change from state S(i) to S(I+1), and the symbol 1 is represented by a change from state S(i) to S(I−1).
Another publication of interest to this invention is “Ternary Physical Protocol for Marilan, A Multiple-Access Ring Local Area Network”, R. J. Kaliman et al., Electrical Engineering Dept., Univ. of Maryland, College Park, Md., pp. 14-20, 1988. FIGS. 4(a) and 4(b) show symbol encoding examples for an exemplary binary sequence and a ternary non-return to zero (NRZ) representation thereof, respectively. In the approach of Kaliman et al. the ring local area network physical layer uses the ternary NRZ code that is suitable for asynchronous transmission, and the code symbols assume values in the balanced ternary set {−1,0,1}. To detect a clock signal, a transition must occur at the end of every bit period and, consequently, two consecutive channel symbols must take different ternary values (as shown in FIGS. 4(a) and 4(b) for the repeats of the binary 1 and binary zero bits).
Neither of the foregoing publications operates with more than three amplitude levels and, hence, they are limited on the amount of data that can be encoded by a symbol and thus the maximum data rate that can be sustained between a transmitter and a receiver.